THA1WE —  WG-E   (21-Jun-18   08:30—10:30)
Chair: P. Forck, GSI, Darmstadt, Germany
Paper Title Page
THA1WE01
New Electron Cloud Instability Mechanism and its Detection and Suppression  
 
  • V.A. Lebedev
    Fermilab, Batavia, Illinois, USA
  • S. A. Antipov
    CERN, Geneva, Switzerland
 
  Fast transverse instability was observed in Recycler proton storage ring (RR). The instability develops within 100 turns and may lead to beam loss. The fast rise suggested that the instability is driven by electron cloud. That was later supported by microwave transmission measurements. In difference to RR the instability was not observed in similar conditions in Main Injector (MI). RR is based on combined function dipoles while MI uses pure dipoles. This difference plays a key role in instability development. The instability dynamics was studied experimentally and with numerical simulations. An analytical model predicts that electrons are trapped in RR dipoles. Numerical simulations show that up to 1% of particles can be trapped. The cloud build-up is exponential with its density limited by space charge. That results in the cloud intensity orders of magnitude greater than in MI. A growth rate of about 30 turns and mode frequency of 0.4 MHz are consistent for observations and PEI simulations. The high intensity batch can be stabilized by low intensity clearing bunch injected behind batch which destroys the trapped electron cloud and prevents its multi-turn accumulation.  
slides icon Slides THA1WE01 [7.328 MB]  
 
THA1WE02 Requirements and Results for Quadrupole Mode Measurements -1
 
  • A. Oeftiger
    CERN, Geneva, Switzerland
 
  Funding: Research supported by the HL-LHC project.
Direct space charge may be quantified, and hence the beam brightness observed, by measuring the quadrupolar beam modes in the CERN Proton Synchrotron (PS). The spectrum of the transverse beam size oscillations (i.e. the quadrupolar beam moment) contains valuable information: the betatron envelope modes and the coherent dispersive mode indicate optics mismatch, while their frequency shifts due to space charge allow a direct measurement thereof. To measure the quadrupolar beam moment we use the Base-Band Q-meter system of the PS which is based on a four electrode stripline pick-up. Past experiments with quadrupolar pick-ups often investigated coasting beams, where the coherent betatron and dispersion modes correspond to single peaks in the tune spectrum. In contrast, long bunched beams feature bands of betatron modes: the mode frequencies shift depending on the transverse space charge strength which varies with the local line charge density. By using the new transverse feedback in the PS as a quadrupolar RF exciter, we measured the quadrupolar beam transfer function. The beam response reveals the distinct band structure of the envelope modes as well as the coherent dispersive mode.
 
slides icon Slides THA1WE02 [7.315 MB]  
 
THA1WE03 BPM Technologies for Quadrupolar Moment Measurements -1
 
  • A. Sounas, M. Gąsior, T. Lefèvre
    CERN, Geneva, Switzerland
 
  Quadrupolar moment measurements based on electromagnetic pick-ups (PU), like BPMs, have attracted particular interest as non-intercepting diagnostics to determine the transverse beam size. Here, the second-order moment, which contains information about the beam size, is extracted from the BPM electrode signals. Despite the simplicity of the concept, quadrupololar measurements have always been challenging in practice. This is related to the fact that the quadrupolar moment constitutes only a very small part of the total PU signal, which is dominated by the contributions of beam intensity and position. In this study we discuss the limitations of absolute quadrupolar measurements if applying traditional BPM technologies, and we propose a new approach to efficiently overcome them via movable PUs. Moreover, we highlight the potential use of BPMs as an emittance measurement system during the energy ramp at synchrotrons by performing differential quadrupolar measurements, which show a remarkably higher accuracy than absolute measurements. Dedicated studies using different types of BPMs in the Large Hadron Collider (LHC) at CERN demonstrated promising results.  
slides icon Slides THA1WE03 [5.299 MB]  
 
THA1WE04 ESS nBLM: Beam Loss Monitors based on Fast Neutron Detection -1
 
  • T. Papaevangelou
    CEA/IRFU, Gif-sur-Yvette, France
  • H. Alves, S. Aune, J. Beltramelli, Q. Bertrand, T. Bey, B. Bolzon, N. Chauvin, M. Combet, D. Desforge, M. Desmons, Y. Gauthier, E. Giner-Demange, A. Gomes, F. Gougnaud, F. Harrault, F. J. Iguaz Gutierrez, T.J. Joannem, M. Kebbiri, C. Lahonde-Hamdoun, P. Le Bourlout, Ph. Legou, O. Maillard, A. Marcel, C. Marchand, Y. Mariette, J. Marroncle, V. Nadot, M. Oublaid, G. Perreu, O. Piquet, B. Pottin, Y. Sauce, J. Schwindling, L. Segui, F. Senée, R. Touzery, G. Tsiledakis, O. Tuske, D. Uriot
    IRFU, CEA, University Paris-Saclay, Gif-sur-Yvette, France
  • I. Dolenc Kittelmann, R.J. Hall-Wilton, C. Höglund, L. Robinson, T.J. Shea, P. Svensson
    ESS, Lund, Sweden
  • V. Gressier
    IRSN, Saint-Paul-Lez-Durance, France
  • K. Nikolopoulos
    Birmingham University, Birmingham, United Kingdom
  • M. Pomorski
    CEA/DRT/LIST, Gif-sur-Yvette Cedex, France
 
  A new type of Beam Loss Monitor (BLM) system is being developed for use in the European Spallation Source (ESS) linac, primarily aiming to cover the low energy part (proton energies between 3-100 MeV). In this region of the linac, typical BLM detectors based on charged particle detection (i.e. Ionization Cham-bers) are not appropriate because the expected particle fields will be dominated by neutrons and photons. Another issue is the photon background due to the RF cavities, which is mainly due to field emission from the electrons from the cavity walls, resulting in brems-strahlung photons. The idea for the ESS neutron sensi-tive BLM system (ESS nBLM) is to use Micromegas detectors specially designed to be sensitive to fast neutrons and insensitive to low energy photons (X and gammas). In addition, the detectors must be insensitive to thermal neutrons, because those neutrons may not be directly correlated to beam losses. The appropriate configuration of the Micromegas operating conditions will allow excellent timing, intrinsic photon back-ground suppression and individual neutron counting, extending thus the dynamic range to very low particle fluxes.  
slides icon Slides THA1WE04 [3.267 MB]